In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of light-emitting elements, a layer containing a substance having a light-emitting property is interposed between a pair of electrodes. Voltage application to this element causes the substance having a light-emitting property to emit light.
Such light-emitting elements are self-luminous elements and have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for backlights, for example; thus, light-emitting elements are thought to be suitable for flat panel display elements. Such light-emitting elements are also highly advantageous in that they can be thin and lightweight. Besides, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in, a film form, they make it possible to provide planar light emission easily; thus, large-area elements using planar light emission can be formed. This is a difficult feature to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent, lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, application of a voltage to a light-emitting element causes injection of electrons and holes from a pair of electrodes into a layer containing the light-emitting organic compound, and thus a current flows. Then, recombination of these carriers (i.e., electrons and holes) brings the organic compound having a light-emitting property into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Note that an excited state formed by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such light-emitting elements, there are a lot of problems that depend on substances, and in order to solve the problems, improvement of the element structures, development of the substances, and the like have been carried out (for example, see Patent Document 1).
The emission wavelength of a light-emitting element is determined by energy difference between a ground state and an excited state, that is, the band gap, of light-emitting molecules included in the light-emitting element. Therefore, a variety of emission colors can be obtained by devising a structure of the light-emitting molecule. With use of light-emitting elements capable of emitting red light, blue light, and green light, which are the three primary colors of light, a full-color light-emitting device can be manufactured.